MODELING OF O-18 TRACER DISTRIBUTION DURING DOUBLE OXIDATION EXPERIMENTS FOR INWARD GROWING SCALES

Quantitative modeling of oxygen tracer (O-18) concentration profiles o btained during double oxidation experiments has been carried out. An e xisting model in the literature, involving combined grain-boundary and lattice diffusion of oxygen with exchange, has been extended to cases where the grain size of the oxide is allowed to vary over its thickne ss. The model predicts tracer profiles in the entire oxide for the cas e of scale growth by inward oxygen diffusion. A computer program has b een developed along with a graphical user interface for easy generatio n and visualization of simulated tracer profiles. The sensitivity of t he simulated profiles to variations in volume (D-u) and grain-boundary (D-b) diffusivities of oxygen in the oxide scale as well as the oxide grain-size variation have been studied. The results indicate that the normalized tracer profiles are sensitive to variations in D-b, althou gh to a lesser extent as compared to similar variations in D-u. Also, incorporation of variable grain size leads to profiles very different from those obtained from a constant oxide grain-size assumption. The c omputer program has been used to fit an experimental profile reported in the literature to obtain values of D-u and D-b. (C) 1995 American I nstitute of Physics.